Not only the sustainability of the success of electro-mobility, but also the development of high-performance cell phones and notebooks make high demands on battery systems. Higher storage capacities, safety and ever-increasing durability are some of the demands they have to live up to. Solid-state lithium-ion batteries are one of the great white hopes in battery research. Compared to conventional lithium-ion batteries with liquid electrolytes, these so-called “all-solid-state“ batteries are ahead of the game as regards safety, operational life span and thermal stability. For this reason, researchers all over the world in the fields of solid-state chemistry, physics and materials science have been under pressure to find suitable, solid-state ion conductors for use in such batteries.
In his doctoral thesis, Viktor Epp, from Graz University of Technology’s Institute of Chemistry and Technology of Materials, looked more closely at the sulphide Li6PSe5Br which was prepared in the well-known working group of Hans-Jörg Deiseroth at the University of Siegen. Using lithium nuclear magnetic resonance spectroscopy, as it is carried out in the CD Laboratory in Martin Wilkening’s group, he came to a remarkable result which confirmed earlier preliminary work: the lithium ions in the investigated sulphide move extremely quickly. This qualifies Li6PS5Br as a front runner among solid-state electrolytes which could be used in solid-state batteries.
“Hopping” atoms: a billion jumps per second
The observed “hopping process” of the lithium ions in Li6PS5Br have proved to be remarkable. With ambient-temperature rates of more than one billion jumps per second, the ions in the investigated sulphide show an extremely high mobility. Such mobility has also been shown in other lithium compounds, however, many of the materials are not only ionically but also electronically conductive – and can thus be excluded as solid-state electrolytes. At first glance, the basic principle of electrochemical energy storage in a lithium-ion battery is relatively easily to understand. During charging and discharging of the battery, the ions move between both poles, thus passing through structurally different materials. In the case of a solid-state lithium-ion battery, a solid, such as a lithium-containing oxide or a sulphide, takes on the role of a conductive electrolyte.
“The more we know about the nature of the charge carrier transport in solids, the more evident it will become, which materials are most suitably for the future development of batteries“, explains Martin Wilkening, who, along with his team in the CD Laboratory, is dedicated to the investigation of microstructures and dynamic processes in new battery materials.